Invasive Non Native Species (INNS) are those which have been transported outside of their natural range and area able to proliferate in their new environment. They are an ongoing issue globally, contributing to habitat loss, species extinction, ecosystem impacts, risks to human health and economic impacts. The island of Ireland has naturally low levels of native species due to its isolation and recent (circa 10,000 years) recolonisation after the last ice age. This has provided a barrier to invasive species, however invasive species have arrived and are continuing to arrive in Ireland through a variety of vectors, almost universally caused by human actions.
There are a wide variety of methods that can result in the introduction and establishment of invasive species, these can be classified as either deliberate or accidental. Deliberate introduction examples can be for biological controls to reduce pest populations, fish to rivers or lakes for sport fishing or ornamental plants for gardens. Accidental introduction occurs unintentionally, these include: as part of ballast water of ships, attached to hulls of ships or when transporting another organism which carries the invasive species. These are often as a result of improper control measures. Aquatic non-native species are thought to be more likely to become invasive, transform ecosystems and cause environmental harm so severe that they are widely considered to be one of the main threats to global biodiversity.
Water is an ideal transport medium for the dispersal of many of these species. Rivers and loughs with their banks and shorelines are amongst the most vulnerable areas to their introduction, spread and impact. The focus for the Loughs Agency is predominantly on aquatic and riparian invasive species as these are a serious threat to our sensitive aquatic habitats. The spread of INNS can also further threaten already endangered native species. In freshwater habitats the introduction of alien species is considered the second leading cause of species extinctions. The issue of climate change response is also affecting the distribution of native species and creating more favourable conditions for non-native species to become more abundant.
There are a multitude of INNS across the UK and Ireland at present, many of them have the potential to cause serious environmental harm. Three species in particular, Japanese Knotweed (Fallopia japonica), Himalayan Balsam (Impatiens grandulifera) and Giant Hogweed (Heracleum mantegazzianum) have become an established threat to the streams and rivers of the Foyle and Carlingford areas. There is a growing body of evidence demonstrating the damaging impacts of INNS. A major consequence as a result of invasive species is the excessive soil erosion along the riparian zone which can have grave consequences for freshwater fish species. Atlantic salmon (Salmo salar) and Trout (Salmo trutta) are reliant upon finding appropriately sized spawning gravel to complete their life cycle. However, Himalayan Balsam will die back in winter time, leaving behind exposed river banks devoid of any natural vegetation. The lack of vegetation on the riparian zone leaves the bank highly susceptible to soil erosion at times of increased flows and floods. Excessive soil erosion will increase the sediment load into the stream reach and can potentially smother the eggs buried in the spawning gravel, starving them of oxygen.
Salmonids produce large numbers of eggs during spawning. Atlantic salmon stocks are at unprecedented low levels and they are also experiencing very high mortality rates during the marine phase of their life cycle. INNS can directly impact on stream habitats through displacement of natural vegetation.
The presence of invasive non-native plants changes the composition of riparian leaf litter by modifying the quality and quantity of detritus available to decomposers and aquatic invertebrates. Some studies have shown that changes in riparian leaf litter can lead to a change in trophic relationships and affect macro-invertebrate community structure within an aquatic ecosystem. INNS can indirectly affect the survival rates of juvenile salmonids and can potentially change the abundance and structure of macro-invertebrate communities. From a fisheries perspective (INNS) pose a very real threat to the long term sustainability of freshwater fish species and their habitats. Many INNS are already becoming well established and a long term strategy is required in order to take appropriate measures to tackle this growing problem.